A video signal typically includes vertical display intervals, or fields, having a plurality of horizontal line intervals, e.g. 262.5 lines per field in NTSC video systems. The beginning of each vertical and horizontal interval is identified by respective vertical and horizontal sync pulses that are included in a composite video signal. During a portion of each vertical interval, information in the video signal may not be intended for display. For example, a vertical blanking interval spans approximately the first 20 horizontal line intervals in each field. In addition, several line intervals adjacent to the vertical blanking period, e.g. line 21, may be within an overscan region of a video display and will not be visible.
The lack of displayed image information during blanking and overscan intervals makes it possible to insert auxiliary information, e.g. teletext or closed caption data, into these intervals. Standards such as Federal Communications Commissions (FCC) Regulations define the format for each type of auxiliary information including the positioning of the information within a vertical interval. For example, the present closed captioning standard (see e.g. 47 CFR .sctn..sctn. 15.119 and 73.682) specifies that digital data corresponding to ASCII characters for closed captioning must be in line 21 of field 1.
The first step in extracting auxiliary video information is to locate the auxiliary information. Various approaches may be used depending on the type of information involved. For example, recognition of teletext data characteristics such as the framing code pattern is a method of locating teletext data. A first step to locating closed caption information is to identify line 21 of field 1. One approach is to count video lines by counting horizontal sync pulses. Examples of line counting approaches may be found in pending International Patent Applications PCT/US/92/04825 and PCT/US/92/04826, filed on 15 Jun. 1992, published on 21 Jan. 1993 (International Publication Numbers WO 93/01680 and WO 93/01681, respectively), and assigned to the assignee of the present application.
After the auxiliary video information is located, the information must be extracted. In the case of digital data, a "data slicer" may be used to convert the video signal into binary data. A data slicer typically operates by comparing the video signal level to a reference level known as the slicing level. For video levels that exceed the slicing level, the comparison produces a logic 1. Video levels that are less than the slicing level produce a logic 0. As an example, closed caption data in line 21 of the video signal may exhibit a signal amplitude range of 0 IRE to 50 IRE. For a signal range of 0 IRE to 50 IRE, a slicing level of 25 IRE would be appropriate.
A constant slicing level may not be adequate for all video signals. Video signal levels may vary depending on the source of the video signal. Utilizing a constant slicing level with varying video signal levels may bias the extracted data undesirably toward logic 0 or logic 1 resulting in erroneous data extraction. For example, if the video signal range is 0 IRE to 20 IRE rather than 0 IRE to 50 IRE, a slicing level of 10 IRE rather than 25 IRE is desirable. If 25 IRE were used as a slicing level for a signal range of 0 IRE to 20 IRE, a logic 1 would never be extracted because the signal never exceeds the slicing level. Thus, it is desirable to adapt the slicing level to the amplitude of the input video signal.
Another possible problem with a constant slicing level is that the operating parameters of components used in a data slicer may vary as a function of temperature, supply voltage, or manufacturer. As an example, characteristics of integrated circuit voltage comparators may vary as a result of temperature and supply voltage changes or different integrated circuit fabrication techniques causing the switching threshold to change. If the amplitude of an auxiliary video data signal is low, changes of component switching thresholds with respect to a fixed slicing level might significantly decrease the accuracy of data extraction. It may be desirable, therefore, to adapt the slicing level to compensate for component variations.
The signal format of auxiliary video data such as closed caption data includes provisions to facilitate an adaptive slicing level function. As specified in the FCC standards, a closed caption signal in line 21 begins after the "back porch" interval of the video signal with a 7 cycle burst of a sinusoidal reference waveform designated the "run-in clock" (RIC). The RIC reference component of the auxiliary video data signal is followed in the latter half of the line 21 interval by a data signal component that represents the actual closed caption data. The closed caption data standard establishes that the amplitude of the RIC signal is identical to the amplitude of the data signal. Thus, the average of the RIC signal amplitude is an appropriate slicing level for the subsequent data signal.
Auxiliary data such as closed-caption data may not exist in all video signals. For example, switching between different video sources may result in switching from one signal having closed caption data to another source that does not. If no auxiliary video data exists, there will be no RIC signal to serve as the basis for modifying the slicing level. Attempting to adapt the slicing level without a RIC signal may produce an inaccurate slicing level.
Another possible problem with certain types of adaptive slicing level systems is that an extended interval (e.g. 1 second) may be required to complete slicing level adjustment. A long slicing level adjustment period may cause an objectionable loss of auxiliary video data while the slicing level is being modified.